The present invention relates to a golf ball which has excellent scratch resistance and ultraviolet ray resistance, in addition to an excellent surface durability whereby generation of face marks by striking with an iron club is decreased, and which is prepared in good coating workability.
Wound golf balls and solid golf balls such as two piece balls are known, wherein a cover made of balata rubber or a thermoplastic resin such as ionomer resin is provided on a core. In both wound golf balls and solid golf balls, paints for golf balls are coated on the surface of the cover in plural layers. In general, such a coating film comprises a bottom layer of a white undercoat paint, one or more intermediate layers of a white intercoat and an outermost layer (surface layer) of a clear top coat.
The functions of the coating film constituting the outermost layer are to impart a gloss to balls to present a fine appearance and to protect the cover. Paints for golf balls used for forming such an outermost layer are required to have properties such as flexibility of coating film which can follow stretch and shrink resulting from deformation of golf balls by external force added when struck, impact resistance which is endurable an impact by a club head, and weatherability which is endurable the use in the outdoors. Also, a good coating workability is required. For example, a two component polyurethane paint is required to have such a coating workability that after mixing two components to form a coating liquid, it has generally a coatable time, namely a pot life, of about 5 hours, and after applying the coating liquid onto golf balls, a time until the coating liquid becomes the state that dust cannot stick, namely dust free drying time, is as short as possible.
Two component polyurethane paints have been conventionally used as paints satisfying such requirements to some extent, e.g., a two component polyurethane paint composed of a polyester polyol or polyether polyol and a polyisocyanate, a two component polyurethane paint composed of an acrylic polyol and a polyisocyanate, and a two component polyurethane paint composed of a polycarbonate diol and a polyisocyanate. However, no paint satisfying all the above-mentioned requirements has been proposed.
Two component polyurethane paints wherein a polyester polyol is used as the polyol component are excellent in film strength, but they have a high hardness and are also inferior in impact resistance and weatherability. Thus, it has also been attempted to use a polyester polyol having a low hydroxyl value or to decrease the amount of a polyisocyanate, thereby lowering the crosslinking density to soften the resulting coating film. These attempts improve the flexibility or softness of the coating film to some extent, but cause another problems that the scratch resistance is lowered and the drying velocity is decreased. A difficulty to improve the weatherability also arises.
In Japanese Patent Publication Kokoku No. 6-93930, it is proposed to use, as a paint for golf balls, a two component polyurethane paint comprising a diisocyanate of non-yellowing type and a hydroxyl group-containing polyester obtained by a reaction of a polycarboxylic acid and a polyhydric alcohol component containing a polyhydric alcohol having an isocyanurate ring. This paint has good surface durability such as scratch resistance and impact resistance, but has a problem that the weatherability is poor.
Two component polyurethane paints wherein a polyether polyol is used as the polyol component have the advantage that coating films having good flexibility and weatherability are easy to be obtained, but have the disadvantage that the scratch resistance is poor. Thus, it is proposed to introduce a rigid group such as aromatic group into the polyether polyol molecular chains, or to further use an active hydrogen compound in order to raise the crosslinking density. It is possible to contemplate an improvement of scratch resistance by these methods, but they cause a problem that the obtained coating films are difficult to follow the stretch and shrink caused by deformation of golf balls at the time of impact, thus being poor in impact resistance.
Two component polyurethane paints wherein an acrylic polyol is used as the polyol component are excellent in weatherability and film strength, but have the disadvantage that the hardness is high and the impact resistance is poor. Thus, it is proposed to use an acrylic polyol having a lower Tg and a low hydroxyl value, or to decrease the amount of a polyisocyanate, or to combine these manners, thereby lowering the crosslinking density to soften the resulting coating film. The flexibility of coating films is improved to some extent by these methods, but a problem of deterioration in scratch resistance arises.
In order to solve the problems as mentioned above of conventional paints for golf balls, Japanese Patent Publication Kokai No. 1-141677 proposes to use, as the golf ball paint, a two component polyurethane paint comprising a polycarbonate diol as the polyol component. This paint has good weatherability and film strength and has also good scratch resistance and impact resistance, and exhibits better properties than those of conventional paints for golf balls. However, it still has a slow drying problem that the dust free drying time is long. If a curing catalyst such as dibutyl tin dilaurate is used in order to shorten the dust free drying time, there arises a problem that the pot life becomes short.
Like this, known golf balls have the problems as mentioned above in the coating films formed on the surface of cover and cannot satisfy all the properties required such as impact resistance, scratch resistance and weatherability. Also, there is a problem of workability in coating golf balls with paints. Thus, it is demanded to develop a golf ball which can satisfy all the properties required for golf ball and can be produced in good coating workability.
It is an object of the present invention to provide a golf ball which can satisfy all properties required for golf balls such as flexibility, film strength and surface durability.
A further object of the present invention is to provide a golf ball having on the surface of a cover a coating film having excellent properties such as scratch resistance, impact resistance, weatherability, flexibility, film strength and elasticity and capable of being formed in good coating workability.
Another object of the present invention is to provide a two component polyurethane paint having excellent properties such as scratch resistance, impact resistance, weatherability, flexibility, elasticity and film strength and having a long pot life and a shortened tack-free or dust-free drying time, and being suitable as a paint for golf balls.
These and other objects of the present invention will become apparent from the description hereinafter.